1. Field
The invention relates to sludge digesters of the gas holding type having a telescoping cover which floats on an envelope of gas generated by decomposing sludge. The cover typically has a top (roof) and a cylindrical side wall. Such digester covers further have ballast members which are generally formed of concrete. These depend into the sludge which has some buoyant effect upon the ballast members to create a differential gas pressure between the submerged and emerged conditions of said ballast members.
2. Technology Background
Ballasted, gas-holding, sludge digesters for digesting municipal waste are well known in the art. U.S. Pat. No. 4,391,705 to Cook, et al. discloses a uniquely ballasted sludge digester of a gas holding type. The ballast members of Cook et al. contain cavities so that sludge fills the cavity to increase the ballast weight as the ballast members emerge from the sludge surface. The Cook et al. sludge digester has been identified by its manufacturer as a "Hydroballast Digester".TM..
Prior to the development of the sludge digesters of the type disclosed in the Cook, et al. patent, it was common to use gas holding covers having solid concrete blocks attached to the lower end of the sideskirt to add extra weight to the cover to increase the pressure of gas contained within the digester. When the concrete block ballasts were submerged in the sludge a buoyant force was exerted by the sludge upon the ballast members according to Archimedes principal. Concrete used in ballast members generally has a density of about 150 pounds per cubic foot. A cubic foot of concrete in a submerged condition in sludge having a specific gravity of about 1.0 has an effective weight which is reduced by the weight of a cubic foot of sludge (about 62.4 pounds per cubic foot). Thus, in a submerged condition one cubic foot of a solid concrete ballast exerts a downward force of about 87.6 pounds.
When the ballast members of a gas holding digester cover emerge from the sludge then the effective weight of the concrete is its normal density, i.e. about 150 lbs/ft.sup.3. Thus, the total weight of the cover is significantly greater when the ballast members are in an emerged condition than when the ballast members are in a submerged condition. This creates a gas pressure differential between the submerged and emerged positions of the ballast. Typically, the operating pressure of the digester is that of the ballast when it is raised from the corbels, but still in a submerged condition. When the ballast members are fully emerged from the sludge the pressure generated is usually at or above the relief valve settings so that typically the ballast members are never fully emerged from the sludge without the relief valves on the cover relieving the pressure of the gas.
Gas storage tanks which employ telescoping covers are known in the petroleum industry wherein volatile liquids such as gasoline are frequently stored in tanks which have a floating cover. In this instance, the cover floats on an envelope of vapors generated by evaporation of gasoline and other volatile liquids. To eliminate evaporation losses which may occur in the "gaps" which exist between the cover and the tank in which the cover telescopes, various types of roof seals have been developed. Exemplary of these roof seals are the seals disclosed in patents to Haupt et al., U.S. Pat. No. 1,919,636, Hills U.S. Pat. No. 4,173,291 and Staber U.S. Pat. No. 2,061,175.
The type of roofs disclosed in Haupt et al. and Hills are ones in which the roof floats directly upon the liquid. The cover disclosed in Staber, however, floats upon an envelope of vapor caused by evaporation of the volatile liquids contained in the tank. The device of Staber is described as a gasometer roof tank and uses a circumferential well located on the exterior of the main tank to hold water to effect a seal between the gases which are typically at a pressure of three inches water column (col. 4, line 22) on the interior and the atmosphere. The device of Staber also provides for the collection of volatile condensate in the well.
Seal troughs have been used because of the volatility of liquids within a tank, e.g. gasoline. The sole purpose of a seal trough, either with a fixed or floating cover, is to provide a system for preventing a vapor or gas from escaping from under the cover to the atmosphere. The depth of such a trough with a floating cover is equivalent to the length of travel of the cover plus a liquid column height equal to the cover pressure which, as indicated in Staber, is often in the range of a few inches water column. The width of a seal trough may be minimal.
In sludge digesters water seals have been used with some fixed covers. Generally, with sludge digester gas holder covers which float on an envelope of gas the sideskirt and ballast have been immersed in the sludge liquid which acts as a seal. Various types of seals have been used with sludge digester floating covers, such as those disclosed in U.S. Pat. Nos. 1,735,461 (Haupt), 1,930,953 (Hampton), 1,919,634 (Haupt et al.) and 4,173,291 (Hills).
Gas-holder telescoping covers of the type disclosed in Kelley, U.S. Pat. No. 3,288,295 and Fisher et al., U.S. Pat. No. 1,989,589 were generally heavier than the more structurally sophisticated covers presently being designed and utilized. Concurrently with the design of lighter covers has been the requirement for increased operating gas pressure. Gas pressures of from six inches of water, and frequently from eight inches or more, up to fifteen inches are relatively common with modern sludge digesters. While the covers of Fisher utilized concrete ballast, the amount or concrete ballast used in a modern gas holding cover is much greater. The need for greater ballasting led to the development of the unique ballast disclosed in Cook et al. Increased ballast weight has resulted in very large concrete ballast members. The use of such large ballast members, including those of the Cook et al. type, within a large sludge tank has generally been readily feasible although concerns over grit accumulation in the Cook et al. type ballast have existed and increased structural support for such large ballasts has been required. Corrosion of ballast support members can be a problem. Corrosive failure of ballast support members can, and has, resulted in some instances of digester operation in the ballast members being dumped in the sludge, causing the cover to tilt and bind.
Gas-holding, sludge digesters have been ballasted in the manner illustrated in Fisher et al. In practice, the sideskirts of such covers are usually constructed quite long so to maintain the ballast in a submerged condition in the fluctuating level of sludge within the digester. The cover of Fisher et al. was ballasted with a concrete ballast member in the form of a continuous ring having a sloped top. The sloped top on the ballast ring was to prevent accumulation or grit and silt on the top surface of the ballast. The ballast member and its supports are generally submerged or partially submerged in the sludge. The sludge contains organic and inorganic liquids and solids and is corrosive and toxic. The immersion of the sideskirt in the sludge exposes the sideskirt, ballast supports, roller guides and the like to gritty, corrosive conditions.
Water troughs external to a fixed cover have been used as seals. Such troughs are situated adjacent the upper edge of the main tank digester usually on the outside of the main tank wall. The cover is fixed to the upper edge of the main tank wall and has a very short skirt which extends downward into the trough. The purpose of the skirt is to cause the gas envelope to be in contact with the water seal. The trough is filled with water to create a water seal to prevent gas on the inside of the cover from escaping to the atmosphere. Such troughs are usually no deeper than about three feet and are about one foot in width.
Launders, which are liquid overflow troughs, are illustrated in U.S. Pat. No. 2,679,477 to Kivari et al. Such troughs are located at the upper lip of tank on the outside surface of the tank wall. These are relatively small in comparison to the tank.
Neither launders, fixed roof seal troughs or seal troughs for telescoping covers are sufficiently large to accommodate the large dimensioned ballast members used in the higher pressure gas-holding sludge digesters presently being constructed.